Matlab failed to open the file. There may be many reasons for this, the file does not exist / the file is not is in the current directory (since you didn't specify a path) / you don't have read access to the file / something else.
Open the file with
[fid, errmsg] = fopen('file.txt', 'a');
assert(fid > 0, 'Failed to open file because: %s', errmsg);
And see what error you get.